A study of coarsening, recrystallization, and morphology of microstructure in Al-Sc-(Zr)-(Mg) alloys
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I.
INTRODUCTION
THE use of dispersoid phases in aluminum alloys is a well-established method of controlling grain growth, recovery, and recrystallization in wrought aluminum alloys. Several reasons exist for the need to control recrystallization. The following are a few examples. Strengthening 5xxx alloys by strain hardening is effective if the unrecrystallized wrought state of the alloy can be maintained. In 2xxx alloys, recrystallization negatively impacts corrosion resistance. Recrystallization in 7xxx alloys increases weld cracking, decreases fracture toughness, and decreases corrosion resistance. The present work seeks to establish the synergistic effects when both Zr and Sc are added to aluminum alloys to control the recrystallization resistance of wrought Al alloys. The behavior of Al3Zr dispersoids has been well studied, and the limits of their effectiveness are well known. It is generally recognized that alloys containing Zr are the most resistant to recrystallization. Although Sc is an experimental addition to Al alloys, the literature has become replete with research describing the effectiveness of Al3Sc dispersoids to retard recrystallization. It may be possible to take advantage of the mutually beneficial properties of each element by forming substitutional Al3(Sc, Zr)-type dispersoids. In practical terms, alloys containing Al3(Sc, Zr) dispersoids have the potential to outperform other alloys with respect to recrystallization resistance. A. Inhibiting Recrystallization with Dispersoids When the alloy is preheated properly, dispersoid phases precipitate via a solid-state reaction to form a distribution of ⬍0.5-m-diameter particles. Dispersoids are known to prevent the motion of subgrain boundaries during annealing,
Y.W. RIDDLE, Research Scientist, is with the Advanced Casting Research Center, Worcester Polytechnic Institute, Worcester, MA 01609-2280. Contact e-mail: [email protected] T.H. SANDERS, Jr., Regents’ Professor, is with the School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245. Manuscript submitted March 8, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
termed “Zener drag,” which inhibits recrystallization.[1] Zener’s theory shows that increasing the volume fraction, decreasing the coarsening kinetics, and maintaining a coherent interface between the dispersoid and matrix all increase the resistance to recrystallization. Also, a homogeneous distribution of dispersoids is essential for good recrystallization resistance. Dispersoids of Al12Mg2Cr, Al20Mn3Cu2, and Al3Zr are presently employed as recrystallization inhibitors in 2xxx and 7xxx aluminum alloys. Of these, Al3Zr is known to be the most potent recrystallization inhibitor.[2,3,4] The literature is replete with references to the commercial potential of adding scandium to wrought aluminum alloys. A recent review article cites much of this information.[5] Scandium forms Al3Sc (L12) dispersoids that impart a high degree of recrystallization resistance in many wrought aluminum alloy systems.
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